Electrically conductive, thermoplastic polymer compounds are of increased practical interest, e.g., for packaging electronic instruments and parts, and for solving a wide range of static discharge, electrostatic dissipation and electromagnetic shielding problems. Often, such compounds are made by mixing solid conductive particles such as carbon black, stainless steel fibers, silver or aluminum flakes or nickel-coated fibers with insulating bulk thermoplastics, for example polystyrene, polyolefins, nylons, polycarbonate, acrylonitrile-butadiene-styrene co-polymers (ABS), and the like.
Recently, there has been an increased interest in replacing carbon black or metal particle-filled compounds of the above-described type with intrinsically electrically conductive polymers and their blends with common insulating polymers. The latter systems are believed to be more economical, easier to process and to exhibit desirable mechanical properties. Among the various conductive polymers, the polyanilines have, in particular, attracted special attention because of their excellent environmental stability and low production costs.
Polyaniline (or abbreviated PANI) is well known in the art, and its synthesis and the preparation of the electrically conductive form of this polymer by, for example, contacting polyanilines with protonic acids resulting in salt complexes has been described in great detail in the prior art cf., for instance, Green, A. G., and Woodhead, A. E., "Aniline-black and Allied Compounds, Part 1," J. Chem. Soc., Vol. 101, pp. 1117 (1912); Kobayashi, et al., "Electrochemical Reactions . . . of Polyaniline Film-Coated Electrodes," J. Electroanl. Chem., Vol. 177, pp. 281-91 (1984); U.S. Pat. Nos. 3,963,498, 4,025,463, 4,983,322 and 5,232,631; Chiang, J.-C. and MacDiarmid, A. G., "Polyaniline: Protonic Acid Doping of the Emeraldine Form to the Metallic Regime", Synthetic Metals, Vol. 13, p. 196 (1986); Salaneck, W. R. et al., "A Two-Dimensional-Surface "State" Diagram for Polyaniline", Synthetic Metals, Vol. 13, p. 297 (1986)!. Typical examples of protonic acids disclosed in the above prior art are HCl, H.sub.2 SO.sub.4, sulfonic acids of the type R.sub.1 --SO.sub.3 H, wherein R.sub.1 stands for an hydrocarbon residue, phosphoric acids, etc. Such acids form salt complexes with polyaniline, which may exhibit electrical conductivities of 10.sup.-3 S/cm or more. Owing to their electrical properties, these so-called "doped" polyanilines or, as used hereinafter, polyaniline salt complexes!, and their blends and compounds with common insulating bulk polymers, are suitable for a variety of the anti-static and shielding applications that are currently served by metal or carbon black filled polymer composition.
Processing of polyanilines in different forms is discussed in several patents and patent applications and a summary of the content of some publications are given below. Major distinctions exist in the techniques employed and final materials properties, depending on whether the polyanilines are processed in the non-doped/non-conducting or conducting salt complex form, and whether the material is in solid or liquid phase during the processing operation. Generally, it is preferred to have the polyaniline in its conductive salt complex form and in fluid phase during processing. The former condition eliminates the need for post-processing doping, which is cumbersome and uneconomical, whereas the latter allows as well for the formation of more homogeneous products as for the production of a greater variety of products in comparison with the limited possibilities encountered in solid state processing.
U.S. Pat. No. 5,006,278 teaches the preparation of a conductive product by mixing a liquid, a doping agent and an undoped polyaniline. The liquid is removed by evaporation. PCT Patent Application WO 89/01694 discloses a processible polyaniline doped with a sulfonic acid. Said polyaniline is useful in processing conducting polymer blends using polyethylene, polypropylene and polyamides as the matrix polymer. According to PCT Patent Specification WO 90/13601 a polymer mixture is prepared by mixing a suitable liquid with a mixture of polyaniline and a multi-sulfonic acid, used as a doping agent, whereafter the liquid is evaporated. According to said specification, the doping is generally carried out at a temperature of 20.degree. to 25.degree. C., and it is disclosed that the doping can be carried out as a heterogeneous reaction, followed by dissolution of the mixture in a suitable solvent. The processing into a final shape is carried out in the presence of a solvent. PCT Patent Application WO 90/10297, EP Patent Application No. 152 632 and U.S. Pat. No. 5,002,700 all disclose the use of dodecylbenzenesulfonic acid as a doping agent for polyaniline. PCT Patent Application WO 90/01775 describes the use of multisulfonic acids as doping agents for polyaniline with the advantage of better thermal stability compared with other sulfonic acids. In the examples of said application, the doping of polyaniline has been carried out in a suspension of polyaniline and the sulfonic acid in an aqueous solution of formic acid. PCT Patent Application WO 93/24554 describes the use of dopant anion substituted with one or more polar group(s) which are preferably hydrogen bonding. PCT Patent Specification WO 93/24555 describes the use of several dopant anions simultaneously, the electrically conducting polymer preferably being in particle form.
The above prior art publications fail to disclose any adequate and economical methods for processing of conductive polyanilines simply in their fluid form; i.e. by, for instance, classic melt processing techniques employed in the thermoplastic polymer industry. Instead, the prior art publications teach quasi-melt processing of compounds comprising conductive polyanilines by mechanically mixing of the components, the conductive polyaniline being in solid phase and only the matrix polymer being in molten form, before shaping the blend into the desired article. Generally, the blends obtained in this manner exhibit varying conductivity, they are very often non-homogeneous and, generally show poor mechanical properties and require a high content of polyaniline particles for the onset of electrical conductivity. In that sense such--erroneously labeled--"melt"--processible conductive thermoplastics closely resemble the well-known carbon black or metal particle-filled systems described above.
A relatively high content of conductive polyaniline is required to reach desirable levels of conductivity in the polyblend; or, in other words, the percolation threshold for the onset of conductivity is relatively high. As used hereinafter, the "percolation threshold" is defined as the weight fraction of conductive material needed to impart a conductivity of 10.sup.-8 S/cm or more to a blend with an insulating matrix polymer. Thus, in the afore-mentioned blends of solid polyaniline particles dispersed in poly(vinyl-chloride) a percolation threshold existed of about at least 13 wt-% of the conductive polyaniline. Such high content of conductive polyaniline particles is not desirable, because it is not economical and, in addition, may substantially alter the mechanical properties of the blend in comparison with those of the pure matrix polymer.
An improved method of making homogeneous conductive polyaniline complexes in a semi-fluid phase, and blends of reduced percolation threshold has been disclosed in EP Patent Application 545 729. According to this application, polyaniline, or derivatives thereof, and an excess of a functionalized organic protonic acid are mechanically mixed. A liquid-like mixture or suspension is obtained which subsequently is thermally solidified between at a temperature in the range of 40.degree. to 250.degree. C. As a result, a dry, solid composition is obtained in the form of a granulate comprising a functionalized protonic acid-doped polyaniline. The latter composition can subsequently be mixed with a thermoplastic polymer and formed into parts of desired shapes using standard polymer melt-processing techniques. Percolation thresholds for the onset of conductivity of parts made according to this method were lower than in the case where solid polyaniline particles were mixed with thermoplastic matrix materials. However, use of the required excess quantity of protonic acid is highly undesirable, as it causes the materials to be acidic, corrosive and hygroscopic, which is undesirable for processing and application of these compositions.
U.S. Pat. No. 5,232,631 discloses solution- and melt-processible polyaniline compositions and blends that exhibit much lower percolation thresholds, sometimes even below 1 wt-%, of conductive polyaniline and a wide variety of non-conducting matrix polymers; such as, polyethylenes, isotactic polypropylene, elastomers, and the like; poly(vinylchloride), polystyrene, polyamides, such as PA 6, PA 12, and the like; poly(methylmethacrylate), polycarbonate, acrylonitrile-butadiene-styrene copolymers (ABS), and the like. However, in the reference the compositions that exhibit a low percolation threshold, invariably, are made from solutions of the conductive polyaniline and the matrix polymers in volatile organic solvents, which is uneconomical and environmentally hazardous and limits the use to products such as film, coatings and fibers. The same reference discloses mixtures of conductive polyaniline and insulating matrix polymers, such as polyethylenes and isotactic polypropylene, which are processed from the melt. However, in the examples shown, the percolation threshold was only slightly lower than for mixtures in which the conductive polyaniline is in the solid form during processing and simply dispersed; cf. Plastics Technology 37 (1991):9 p. 19-20. In addition, U.S. Pat. No. 5,232,631, and EP Patent Application No. 545,729 teach that, in order to process conductive polyaniline from the melt and affect even a modest reduction in percolation threshold, an excess of protonic acid is needed, which renders the final blend undesirably acidic. As was pointed out above, the latter generally is unacceptable from a processing, application and environmental point of view. The excess acid, of course, can be removed, but this process is tedious and uneconomical, and limits the scope of the products that can be manufactured.
EP Patent Application No. 582,919 teaches the use of reaction products of metal compounds and protonic acids as plasticizers for conducting polyanilines and for reducing the percolation threshold for the onset of conductivity in blends comprising bulk polymers; and for neutralizing acidic, protonated polyaniline compositions. This reference specifically teaches the use of the reaction product between zinc oxide and dodecylbenzenesulfonic acid as a plasticizer and neutralization agent for conducting forms of polyaniline and blends thereof. The use of the afore-mentioned reaction product in conducting polyaniline compositions, however, involves additional process steps that are uneconomical and reduce the environmental stability of the final materials. In addition, manufacturing of the above metal oxide-acid reaction products involves large amounts of highly corrosive and highly hygroscopic acids, which is unacceptable from a production and processing point of view.
Generally, the use of plasticizers are well known to those skilled in the art of polymer technology, and, not surprisingly, has also been employed in conductive polymer processing, cf. EP Patent Application No. 497,514, U.S. Pat. No. 5,171,478 and PCT Patent Application WO 92/22911. Typically, plasticizers are used to enhance flow, and/or reduce the viscosity of polymeric materials during processing; and generally form a part of the polymer composition. Along these well-known uses, EP 497,514 cited supra teaches the use of highly polar, ester-free plasticizer to plasticize the thermoplastic component of polymer blends comprising polyaniline. According to the reference, the plasticizer is selected to facilitate the flow only of the thermoplastic matrix, such as poly(vinyl chloride); and not to induce flow, or dissolve the polyaniline.
U.S. Pat. No. 5,171,478, teaches the use of an extraordinary wide variety of chemical species termed plasticizers, ranging from water, p-toluene sulfonic acid to synthetic waxes and fluorinated hydrocarbons, to assist in thermally induced chain coupling of polyaniline which, according to the text, should be in the solid state. Clearly, as is evident from the latter statement, and judging from their chemical nature, the suggested "plasticizing" species are not intended to facilitate flow, or dissolve the polyaniline component in the compositions.
Conventional use of commercially available plasticizers is discussed in WO 92/22911. For example, Mesamoll (Bayer) was employed in a blend of conductive polyaniline and poly(vinyl chloride). However, Mesamoll is not a solvent for conductive polyaniline, and much like in the results in the above cited Plastics Technology 37 (1991):9 p. 19-20 a percolation threshold existed of more than 10% w/w of the conductive polyaniline.
Thus, in summary, there still clearly exists a need for electrically conductive polyaniline compositions which can be processed in the fluid state and which have low percolation thresholds. In particular, there is a need for fluid-phase processible electrically conductive polyaniline composition which are doped with non-functionalized counter-ions by using mineral acids, such as sulfuric acid or hydrochloric acid, because such non-funtionalized counter-ions will allow for simplified processing of the compositions and polymer blends prepared from the compositions.